The present invention relates to a method and a circuit for detecting the presence of a television, a video playback and/or recording unit, such as a VCR, or other such device on an output of a video digital to analog converter (DAC). Hereinafter such televisions, video recording and/or playback units and other such devices are referred to as xe2x80x9ctelevision devicesxe2x80x9d.
With the increasing merging of computer and television technologies, and in particular, the application of digital technology to the transmission and relaying of video signals, video signal output devices are required for converting a digital video or other such signal to an analog output signal which should be suitable for inputting to a television device. Such video signal output devices comprise one or more video DACs, for example, a composite DAC, a luminance DAC, a chrominance DAC and other DACs. It is desirable that where the output of a composite or a luminance DAC is not connected to a television device that the video DAC should be powered down in order to reduce power consumption.
Methods and circuits for determining the presence or absence of a television device connected to an output of a video DAC are known. Such circuits and methods require that the video signal through the video DAC be interrupted, and during the interruption, a constant current signal is forced onto the video DAC output. The voltage on the video DAC output is compared with a reference voltage for determining the presence or absence of a television device on the video DAC output. The results of the comparison are stored, and can be read by the system controller of the video signal output device, and appropriate action taken. Once the comparison of the voltage on the video DAC output with a reference voltage has been made the video DAC can then revert to outputting the video signal. The interruption of the video signal for permitting the constant current signal to be forced onto the video DAC output is undesirable, since in general, it introduces a flicker, which in many cases may be visible on a television output screen. Additionally, any interruption in a video signal causes problems when a video signal is being recorded in a video unit. This, is undesirable.
There is therefore a need for a method and circuit for detecting the presence of a television device on the output of a video DAC.
According to the invention there is provided a method for detecting the presence of a television device of the type hereinbefore defined on the output of a video digital to analog converter (DAC), the method comprising the steps of:
applying a video output signal from the DAC to a control impedance means for developing a voltage across the control impedance means,
comparing the voltage developed across the control impedance means with a reference voltage when the video output signal is of a known level for determining the presence of a television device, and
outputting a flag signal for indicating the presence of a television device in response to the presence of a television device being detected.
Preferably, the comparison of the developed voltage across the control impedance means with the reference voltage is made at a predetermined time during a blanking interval, for example, a vertical blanking interval of the video output signal. The blanking interval of a video signal is an interval which contains no video picture data, but rather control signals such as synchronising pulses for synchronising the lines and/or frame of the picture. The vertical blanking interval is the interval which contains serration and equalisation pulses of the video signal.
Advantageously, the predetermined time at which the comparison of the developed voltage across the control impedance means with the reference voltage is made coincides with a portion of a pulse signal in the vertical blanking interval of the video output signal between a rising edge and an adjacent falling edge of the pulse signal.
In one embodiment of the invention the pulse signal is an equalisation pulse in the vertical blanking interval of the video output signal.
Advantageously, the comparison of the developed voltage across the control impedance means with the referenced voltage is made for a first predetermined period. Preferably, the predetermined period lies in the range of 20 clock cycles to 100 clock cycles, where a line of the video output signal is equivalent to 1,716 clock cycle, and advantageously, the predetermined period is approximately 50 clock cycles.
Preferably, the comparison of the developed voltage across the control impedance means with the reference voltage is made at predetermined intervals of the video output signal.
Advantageously, each predetermined interval at which the comparison of the developed voltage across the control impedance means with the reference voltage is made corresponds to a predetermined number of fields of the video output signal.
It is preferable that the comparison of the developed voltage across the control impedance means with the reference voltage is made at least once every eight fields of the video output signal, and preferably, once every two fields of the video output signal.
Preferably, the impedance value of the control impedance means is selected to be a function of the value of the impedance value of the television device, and ideally, the impedance of the control impedance means is selected to be similar to the value of the impedance of the television device.
In one embodiment of the invention the control impedance means is provided by a control resistor connected across the output of the video DAC and ground.
Ideally, the value of the reference voltage is selected so that the voltage developed across the control impedance means lies to one side of the reference voltage when a television device is connected to the output of the video DAC, and lies to the other side of the reference voltage in the absence of a television device.
In another embodiment of the invention the flag signal is read, and the video DAC is powered down in response to the flag signal indicating the absence of a television device on the video DAC output.
Preferably, on being powered down the video DAC is powered up for second predetermined periods at the predetermined times at the predetermined intervals for outputting a portion of the video output signal to the control impedance means for developing the voltage across the control impedance means for comparison thereof with the reference voltage, and powering up the video DAC in response to the flag signal indicating the presence of a television device on the video DAC output.
Additionally, the invention provides a detector circuit for detecting the presence of a television device of the type hereinbefore defined on the output of a video DAC, the circuit comprising:
a control impedance means for connecting to the video DAC output so that a voltage is developed across the control impedance means by a video output signal from the video DAC,
a comparing means for comparing the developed voltage across the control impedance means with a reference voltage when the video output signal is of a known level for determining the presence of a television device, and
an output means responsive to the comparing means for outputting a flag signal for indicating the presence of a television device.
In one embodiment of the invention a timing means is provided for outputting a control signal for initiating the comparison of the developed voltage across the control impedance means with the reference voltage at a predetermined time during a blanking interval, for example, a vertical blanking interval of the video output signal, the timing means being synchronised with a video input signal to the video DAC.
Preferably, the predetermined time at which the timing means outputs the control signal is at a time which coincides with a portion of a pulse signal in the vertical blanking interval of the video output signal between a rising edge and an adjacent falling edge of the pulse signal.
In one embodiment of the invention the pulse signal is an equalisation pulse in the vertical blanking interval of the video signal.
In another embodiment of the invention the timing means outputs the control signal for a first predetermined period during which the comparison of the developed voltage across the control impedance means with the reference voltage is made. Preferably, the predetermined period lies in the range of 20 clock cycles to 100 clock cycles where the length of a line of the video output signal is equivalent to 1,716 clock cycles. Advantageously, the predetermined period is approximately 50 clock cycles.
Preferably, the timing means outputs the control signal at predetermined intervals of the video output signal.
Advantageously, each predetermined interval corresponds to a predetermined number of fields of the video output signal. Preferably, the timing means outputs the control signal at least once every eight fields of the video output signal, and preferably, at least once every two fields of the video output signal.
In one embodiment of the invention the impedance value of the control impedance means is a function of the value of the impedance of the television device, and preferably, the impedance value of the control impedance means is similar to the value of the impedance of the television device.
In one embodiment of the invention the control impedance means is a control resistor and is adapted for connecting between the output of the video DAC and ground.
Ideally, the value of the reference voltage is selected so that the developed voltage across the control impedance means lies on one side of the reference voltage when a television device is connected to the output of the video DAC, and lies to the other side of the reference voltage in the absence of a television device.
In one embodiment of the invention a control means is provided for powering down the video DAC in response to the flag signal indicating the absence of a television device on the output of the video DAC.
Preferably, the control means in response to the control signals from the timing means powers up the video DAC for second predetermined periods at the predetermined times at the predetermined intervals for outputting a portion of the video signal to the control impedance means for developing the voltage across the control impedance means for comparison thereof with the reference voltage by the comparing means, the control means being responsive to the flag signal indicating the presence of a television device on the output of the video DAC for powering up the video DAC.
The invention further provides a video DAC comprising a detector circuit for detecting the presence of a television device of the type hereinbefore defined on the output of the video DAC, the circuit comprising:
a control impedance means for connecting to the video DAC output so that a voltage is developed across the control impedance means by a video output signal from the video DAC,
a comparing means for comparing the developed voltage across the control impedance means with a reference voltage when the video output signal is of a known level for determining the presence of a television device, and
an output means responsive to the comparing means for outputting a flag signal for indicating the presence of a television device.
The advantages of the invention are many. By virtue of the fact that the method and the detector circuit use the voltage developed across the control impedance means by the actual video output signal for comparison with the reference voltage, there is no danger of interruption, interference or distortion of the video signal, and accordingly, the comparison of the voltage developed across the control impedance means with the reference voltage is entirely unintrusive and has no visible effect on the television output screen, nor does it have any effect on recording of a video output signal on a VCR. Additionally, by virtue of the fact that the comparison of the voltage developed across the control impedance means with the reference voltage is made when the video output signal is of a known level, for example, during a single pulse in the vertical blanking interval of the video output signal, the value of the voltage developed across the control impedance means can have only one of two values, one for indicating the presence of a television device, and the other for indicating the absence of a television device. By selecting the reference voltage to lie between these two voltage levels, an accurate and simple means for indicating the presence or absence of a television device is provided.
A further important advantage of the invention is that there is no need for the generation of an additional signal to be applied to the video DAC output, since the detection of a television device is carried out by using the video output signal to develop a voltage across the control impedance means. Accordingly, the method and the detector circuit according to the invention for detecting the presence of a television device on the output of a video DAC is a relatively simple and inexpensive method and circuit, which may be implemented with any video DAC at relatively low cost.
The circuit and method for detecting the presence of a television, video unit or other device on an output of a video DAC may be implemented in any video signal output device, such as, for example, a digital video encoder for encoding digital video signals, for example, satellite video signals, video output data from computers and the like.
A particularly important advantage of the invention is achieved when the method and circuit comprise a means for powering up and powering down the video DAC in response to the flag signal since this eliminates the need for manual intervention for powering up and powering down the video DAC depending on whether or not a television device is connected to the video DAC output.
The invention will be more clearly understood from the following description of some preferred embodiments thereof which are given by way of example only with reference to the accompanying drawings.